Content distributed by a network operator to clients, including satellite radio, streaming media over the Internet, digital cable television programming, and so on can be configured to contain advertisement insertion opportunities that are referred to as “avails.” Traditionally, avails have a corresponding duration that is fixed. Accordingly, avails of this type can be filled using a single advertisement that has a duration that is substantially the same as the duration of the avail. Ads from advertisers, though, can have durations of varying length. Traditional avails that are limited to being filled using ads of a fixed duration can reduce flexibility to place ads and make it difficult to optimize allocation of multiple ads to a large inventory of avails.